1. Field of the Invention
This invention relates to silicon epitaxial growth processes, and more particularly, to selective epitaxial growth processes for integrated circuit fabrication.
2. Description of the Prior Art
Selective epitaxial growth on silicon wafers is used to form isolated regions of silicon in which devices may be formed, such as bipolar and field-effect transistors. The primary advantages gained by having isolated regions are the reduced parasitic capacitive coupling of the devices to the substrate and the excellent inter-isolation of devices formed in different regions. These advantages allow for higher speed and higher operating voltages than attainable in circuits fabricated using prior-art isolation techniques, such as the local oxidation of silicon process (LOCOS) widely used in CMOS circuit fabrication.
In general, the formation of the selective epitaxial regions or layers on a wafer involves opening windows in an oxide layer to expose the underlying substrate. This is done with conventional photolithographic techniques and anisotropic etching, such as a reactive ion etch (RIE). The wafer is then placed in a reactor and silicon is grown, and the desired dopant is simultaneously incorporated therewith, to form the epitaxial regions on the exposed substrate. One such technique is described in "Selective silicon epitaxial growth for device-isolation technology", by A. Ishitani, et al., in Microelectronic Engineering, Vol. 4, 1986, pp. 3-33. However, the described processes allow for only about 0.6 micron of silicon growth before significant faceting (here, 0.1 micron or more) occurs on the major surface of the epitaxial region. As will be discussed in more detail below, faceting is the formation of another growth plane at a different angle from the major surface of the region, typically forming at the sides of the region where it meets the wall of the oxide layer. The plane of the facet is along a different crystallographic plane than the major surface of the epitaxial region. As will be discussed in more detail below, such faceting reduces the amount of area on the major surface of the epitaxial region available for fabrication of a device for a given region size. Further, the limitation of 0.6 micron of epitaxial region thickness limits the operating voltage of the devices formed in the region before break-down of the device to the substrate occurs.
Another difficulty with the above-referenced selective growth process is the need to modify the process depending on the amount of silicon substrate area exposed on the wafer for the selective epitaxial layers to be grown. These variations are necessary so that the growth rate and, hence, the degree of facet formation, is controlled to give the desired thickness of the layer before significant faceting occurs. A consequence of this process variation is the inability of one reactor using this process to simultaneously grow consistent thickness epitaxial regions on wafers having different amounts of exposed silicon thereon.